60" radius is recommended by the Layout Design Special Interest Group for good appearance and full functioning of couplers on curves for 12" (actual length) long cars.
36" radius and higher is recommended as the minimum by the LDSIG rule of thumb. 36" radius will allow you to use normal body mounted couplers, and full details underneath without the trucks binding on the details in curves. Couplers may couple but will not uncouple automatically on curves. Diaphrams on the vestibules will probably work. Long trains will not string line on a helix.
30" radius is about the minimum for non-pivoting body-mounted couplers on that length of car. Some underbody details may interfere with trcuk swiveling.
24" radius will require either truck-mounted couplers or special pivoting body mounts. Diaphrams will not match up. Overhang is becoming excessive. Some underbody details will likely have to be cut away. Cars cannot be coupled or uncoupled on curves. Testing of each car becomes necessary.
Most unmodified commercial models of 85 ft cars will not do 18" radius. Truck-mounted couplers and adjustment of underbody detail to allow free pivoting of trucks is critical. Individual testing of each car and combinations of cars and engines is required. String lining of long trains where any car is not free-rolling is likely. Overhang is such that the outside of the car body is practically inside the inner rail at the center. Reliable and derailment-free operation becomes quite difficult to achieve. Only certain engines will work and cars will likely have to be placed in a specific order that has been tested.
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